SREBP-1 integrates the actions of thyroid hormone, insulin, cAMP, and medium-chain fatty acids on ACCalpha transcription in hepatocytes

Understanding the Roles of Selenium on Thyroid Hormone-Induced Thermogenesis in Adipose Tissue

Brown adipose tissue (BAT) and white adipose tissue (WAT) are known to regulate lipid metabolism. A lower amount of BAT compared to WAT, along with adipose tissue dysfunction, can result in obesity. Studies have shown that selenium supplementation protects against adipocyte dysfunction, decreases WAT triglycerides, and increases BAT triiodothyronine (T3). In this review, we discuss the relationship between selenium and lipid metabolism regulation through selenoprotein deiodinases and the role of deiodinases and thyroid hormones in the induction of adipose tissue thermogenesis. Upon 22 studies included in our review, we found that studies investigating the relationship between selenium and deiodinases demonstrated that selenium supplementation affects the iodothyronine deiodinase 2 (DIO2) protein and the expression of its associated gene, DIO2, proportionally. However, its effect on DIO1 is inconsistent while its effect on DIO3 activity is not detected. Studies have shown that the activity of deiodinases especially DIO2 protein and DIO2 gene expression is increased along with other browning markers upon white adipose tissue browning induction. Studies showed that thermogenesis is stimulated by the thyroid hormone T3 as its activity is correlated to the expression of other thermogenesis markers. A proposed mechanism of thermogenesis induction in selenium supplementation is by autophagy control. However, more studies are needed to establish the role of T3 and autophagy in adipose tissue thermogenesis, especially, since some studies have shown that thermogenesis can function even when T3 activity is lacking and studies related to autophagy in adipose tissue thermogenesis have contradictory results.

Selenomethionine alleviates environmental heat stress induced hepatic lipid accumulation and glycogen infiltration of broilers via maintaining mitochondrial and endoplasmic reticulum homeostasis

With the increasing of global mean surface air temperature, heat stress (HS) induced by extreme high temperature has become a key factor restricting the poultry industry. Liver is the main metabolic organ of broilers, HS induces liver damage and metabolic disorders, which impairs the health of broilers and affects food safety. As an essential trace element for animals, selenium (Se) involves in the formation of antioxidant system, and its biological functions are generally mediated by selenoproteins. However, the mechanism of Se against HS induced liver damage and metabolic disorders in broilers is inadequate. Therefore, we developed the chronic heat stress (CHS) broiler model and investigated the potential protection mechanism of organic Se (selenomethionine, SeMet) on CHS induced liver damage and metabolic disorders. In present study, CHS caused liver oxidative damage, and induced hepatic lipid accumulation and glycogen infiltration of broilers, which are accompanied by mitochondrial dysfunction, abnormal mitochondrial tricarboxylic acid (TCA) cycle and endoplasmic reticulum (ER) stress. Dietary SeMet supplementation increased the hepatic Se concentration and exhibited protective effects via promoting the expression of selenotranscriptome and several key selenoproteins (GPX4, TXNRD2, SELENOK, SELENOM, SELENOS, SELENOT, GPX1, DIO1, SELENOH, SELENOU and SELENOW). These key selenoproteins synergistically improved the antioxidant capacity, and mitigated the mitochondrial dysfunction, abnormal mitochondrial TCA cycle and ER stress, thus recovered the hepatic triglyceride and glycogen concentration. What's more, SeMet supplementation suppressed lipid and glycogen biosynthesis and promoted lipid and glycogen breakdown in liver of broilers exposed to CHS though regulating the AMPK signals. Overall, our present study reveals a potential mechanism that Se alleviates environment HS induced liver damage and glycogen and lipid metabolism disorders in broilers, which provides a preventive and/or treatment measure for environment HS-dependent hepatic metabolic disorders in poultry industry.

Gallstone Disease and Its Correlation With Thyroid Disorders: A Narrative Review

Over the years, several studies have revealed an important link between thyroid disorders and gallstone disease. According to these studies, hypothyroidism and hyperthyroidism are associated with cholesterol gallstone disease. This association between thyroid hormone disorders and cholesterol gallstone disease is due to the importance of thyroid hormones on cholesterol synthesis, bile functioning and content, and gallbladder motility. Several genes and receptors have been found on the thyroid gland, liver, and gallbladder to verify this association. These genes affect thyroid hormone secretion, lipid metabolism, and bile secretion. Defects in these various gene expression and protein functions lead to bile duct diseases. Other causes that lead to cholesterol gallstone disease are supersaturation of the bile with cholesterol and impaired gallbladder motility, which leads to bile stasis. This article has discussed these factors in detail while highlighting the association between thyroid hormones and cholesterol gallstone disease.

Selenoproteins synergistically protect porcine skeletal muscle from oxidative damage via relieving mitochondrial dysfunction and endoplasmic reticulum stress

BACKGROUND

The skeletal muscle of pigs is vulnerable to oxidative damage, resulting in growth retardation. Selenoproteins are important components of antioxidant systems for animals, which are generally regulated by dietary selenium (Se) level. Here, we developed the dietary oxidative stress (DOS)-inducing pig model to investigate the protective effects of selenoproteins on DOS-induced skeletal muscle growth retardation.

RESULTS

Dietary oxidative stress caused porcine skeletal muscle oxidative damage and growth retardation, which is accompanied by mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and protein and lipid metabolism disorders. Supplementation with Se (0.3, 0.6 or 0.9 mg Se/kg) in form of hydroxy selenomethionine (OH-SeMet) linearly increased muscular Se deposition and exhibited protective effects via regulating the expression of selenotranscriptome and key selenoproteins, which was mainly reflected in lower ROS levels and higher antioxidant capacity in skeletal muscle, and the mitigation of mitochondrial dysfunction and ER stress. What's more, selenoproteins inhibited DOS induced protein and lipid degradation and improved protein and lipid biosynthesis via regulating AKT/mTOR/S6K1 and AMPK/SREBP-1 signalling pathways in skeletal muscle. However, several parameters such as the activity of GSH-Px and T-SOD, the protein abundance of JNK2, CLPP, SELENOS and SELENOF did not show dose-dependent changes. Notably, several key selenoproteins such as MSRB1, SELENOW, SELENOM, SELENON and SELENOS play the unique roles during this protection.

CONCLUSIONS

Increased expression of selenoproteins by dietary OH-SeMet could synergistically alleviate mitochondrial dysfunction and ER stress, recover protein and lipid biosynthesis, thus alleviate skeletal muscle growth retardation. Our study provides preventive measure for OS-dependent skeletal muscle retardation in livestock husbandry.

Thyroid hormone Beta receptor agonists for treatment of kidney disease: A promising agent?

BACKGROUND

Chronic kidney disease is a common disorder affecting a significant portion of the adult population with high mortality and morbidity. Obesity and hyperlipidemia are prevalent in chronic kidney disease, and they may trigger fat accumulation in renal parenchyma and eventually fatty kidney. Chronic kidney disease and fatty kidney are also strongly associated with nonalcoholic fatty liver disease. Because they both lead to detrimental effects on organ function, they both need to be treated effectively to improve the outcome.

AIM

In this narrative review, we have hypothesized that thyroid hormone beta receptor agonists, a novel drug group, may potentially be beneficial in the management of chronic kidney disease due to its promising outcomes among patients with nonalcoholic fatty liver disease, a condition sharing multiple common underlying pathophysiological mechanisms.

RESULTS AND CONCLUSION

Thyroid hormone beta receptors are abundantly expressed in liver and kidney tissues, while both nonalcoholic fatty liver disease and chronic kidney disease share various similar pathophysiological mechanisms and triggers. Therefore, thyroid hormone beta receptor agonists may become a promising tool in the management of patients with chronic kidney disease.

Effects of Thyroid Hormones on Lipid Metabolism Pathologies in Non-Alcoholic Fatty Liver Disease

The typical modern lifestyle contributes to the development of many metabolic-related disorders, as exemplified by metabolic syndrome. How to prevent, resolve, or avoid subsequent deterioration of metabolic disturbances and the development of more serious diseases has become an important and much-discussed health issue. Thus, the question of the physiological and pathological roles of thyroid hormones (THs) in metabolism has never gone out of fashion. Although THs influence almost all organs, the liver is one of the most important targets as well as the hub of metabolic homeostasis. When this homeostasis is out of balance, diseases may result. In the current review, we summarize the common features and actions of THs, first focusing on their effects on lipid metabolism in the liver. In the second half of the review, we turn to a consideration of non-alcoholic fatty liver disease (NAFLD), a disease characterized by excessive accumulation of fat in the liver that is independent of heavy alcohol consumption. NAFLD is a growing health problem that currently affects ~25% of the world’s population. Unfortunately, there are currently no approved therapies specific for NAFLD, which, if left uncontrolled, may progress to more serious diseases, such as cirrhosis or liver cancer. This absence of effective treatment can also result in the development of non-alcoholic steatohepatitis (NASH), an aggressive form of NAFLD that is the leading cause of liver transplantation in the United States. Because THs play a clear role in hepatic fat metabolism, their potential application in the prevention and treatment of NAFLD has attracted considerable research attention. Studies that have investigated the use of TH-related compounds in the management of NAFLD are also summarized in the latter part of this review. An important take-home point of this review is that a comprehensive understanding of the physiological and pathological roles of THs in liver fat metabolism is possible, despite the complexities of this regulatory axis—an understanding that has clinical value for the specific management of NAFLD.

Hepatocytic Activating Transcription Factor 3 Protects Against Steatohepatitis via Hepatocyte Nuclear Factor 4α

Activating transcription factor 3 (ATF3) has been shown to play an important role in HDL metabolism; yet, the role of hepatocytic ATF3 in the development of steatohepatitis remains elusive. Here we show that adenoassociated virus-mediated overexpression of human ATF3 in hepatocytes prevents diet-induced steatohepatitis in C57BL/6 mice and reverses steatohepatitis in db/db mice. Conversely, global or hepatocyte-specific loss of ATF3 aggravates diet-induced steatohepatitis. Mechanistically, hepatocytic ATF3 induces hepatic lipolysis and fatty acid oxidation and inhibits inflammation and apoptosis. We further show that hepatocyte nuclear factor 4α (HNF4α) is required for ATF3 to improve steatohepatitis. Thus, the current study indicates that ATF3 protects against steatohepatitis through, at least in part, hepatic HNF4α. Targeting hepatic ATF3 may be useful for treatment of steatohepatitis.

Hepatocyte Nuclear Factor 4α Prevents the Steatosis-to-NASH Progression by Regulating p53 and Bile Acid Signaling (in mice)

BACKGROUND AND AIMS

Hepatocyte nuclear factor 4α (HNF4α) is highly enriched in the liver, but its role in the progression of nonalcoholic liver steatosis (NAFL) to NASH has not been elucidated. In this study, we investigated the effect of gain or loss of HNF4α function on the development and progression of NAFLD in mice.

APPROACH AND RESULTS

Overexpression of human HNF4α protected against high-fat/cholesterol/fructose (HFCF) diet-induced steatohepatitis, whereas loss of Hnf4α had opposite effects. HNF4α prevented hepatic triglyceride accumulation by promoting hepatic triglyceride lipolysis, fatty acid oxidation, and VLDL secretion. Furthermore, HNF4α suppressed the progression of NAFL to NASH. Overexpression of human HNF4α inhibited HFCF diet-induced steatohepatitis in control mice but not in hepatocyte-specific p53-/- mice. In HFCF diet-fed mice lacking hepatic Hnf4α, recapitulation of hepatic expression of HNF4α targets cholesterol 7α-hydroxylase and sterol 12α-hydroxylase and normalized hepatic triglyceride levels and attenuated steatohepatitis.

CONCLUSIONS

The current study indicates that HNF4α protects against diet-induced development and progression of NAFLD by coordinating the regulation of lipolytic, p53, and bile acid signaling pathways. Targeting hepatic HNF4α may be useful for treatment of NASH.

Thyroid Hormone Signaling and the Liver

Thyroid hormone (TH) plays a critical role in maintaining metabolic homeostasis throughout life. It is well known that the liver and thyroid are intimately linked, with TH playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Indeed, patients with hypothyroidism have abnormal lipid panels with higher levels of low-density lipoprotein levels, triglycerides (triacylglycerol; TAG), and apolipoprotein B levels. Even in euthyroid patients, lower serum-free thyroxine levels are associated with higher total cholesterol levels, LDL, and TAG levels. In addition to abnormal serum lipids, the risk of nonalcoholic fatty liver disease (NAFLD) increases with lower free thyroxine levels. As free thyroxine rises, the risk of NAFLD is reduced. This has led to numerous animal studies and clinical trials investigating TH analogs and TH receptor agonists as potential therapies for NAFLD and hyperlipidemia. Thus, TH plays an important role in maintaining hepatic homeostasis, and this continues to be an important area of study. A review of TH action and TH actions on the liver will be presented here.

Hepatic Forkhead Box Protein A3 Regulates ApoA-I (Apolipoprotein A-I) Expression, Cholesterol Efflux, and Atherogenesis

OBJECTIVE

To determine the role of hepatic FOXA3 (forkhead box A3) in lipid metabolism and atherosclerosis. Approach and Results: Hepatic FOXA3 expression was reduced in diabetic or high fat diet-fed mice or patients with nonalcoholic steatohepatitis. We then used adenoviruses to overexpress or knock down hepatic FOXA3 expression. Overexpression of FOXA3 in the liver increased hepatic ApoA-I (apolipoprotein A-I) expression, plasma HDL-C (high-density lipoprotein cholesterol) level, macrophage cholesterol efflux, and macrophage reverse cholesterol transport. In contrast, knockdown of hepatic FOXA3 expression had opposite effects. We further showed that FOXA3 directly bound to the promoter of the Apoa1 gene to regulate its transcription. Finally, AAV8 (adeno-associated virus serotype 8)-mediated overexpression of human FOXA3 in the hepatocytes of Apoe^-/- (apolipoprotein E-deficient) mice raised plasma HDL-C levels and significantly reduced atherosclerotic lesions.

CONCLUSIONS

Hepatocyte FOXA3 protects against atherosclerosis by inducing ApoA-I and macrophage reverse cholesterol transport.

Lipocalin-2 Protects Against Diet-Induced Nonalcoholic Fatty Liver Disease by Targeting Hepatocytes

Hepatocytes are the major source of hepatic lipocalin-2 (LCN2), which is up-regulated in response to inflammation, injury, or metabolic stress. So far, the role of hepatocyte-derived LCN2 in the development of nonalcoholic fatty liver disease (NAFLD) remains unknown. Herein we show that overexpression of human LCN2 in hepatocytes protects against high fat/high cholesterol/high fructose (HFCF) diet-induced liver steatosis and nonalcoholic steatohepatitis by promoting lipolysis and fatty acid oxidation (FAO) and inhibiting de novo lipogenesis (DNL), lipid peroxidation, and apoptosis. LCN2 fails to reduce triglyceride accumulation in hepatocytes lacking sterol regulatory element-binding protein 1. In contrast, Lcn2-/- mice have defective lipolysis, increased lipid peroxidation and apoptosis, and exacerbated NAFLD after being fed an HFCF diet. In primary hepatocytes, Lcn2 deficiency stimulates de novo lipogenesis but inhibits FAO. Conclusion: The current study indicates that hepatocyte LCN2 protects against diet-induced NAFLD by regulating lipolysis, FAO, DNL, lipid peroxidation, and apoptosis. Targeting hepatocyte LCN2 may be useful for treatment of NAFLD.

Functional Analysis of Promoters of Genes in Lipid Metabolism and Their Transcriptional Response to STAT3 under Leptin Signals

We characterized the promoters of target genes of the signal transducer and activator of transcription 3, STAT3 (carnitine palmitoyltransferase I, CPT Iα1b, acetyl-CoA carboxylase alpha, ACCα; fatty acid synthase, FAS; and peroxisome proliferator-activated receptor gamma, PPARγ) in a teleost Pelteobagrus fulvidraco. Binding sites of STAT3 were predicted on these promoters, indicating that STAT3 probably mediated their transcriptional activities. Leptin had no effect on the activity of ACCα and PPARγ promoters, but increased CPT Iα1b promoter activity and decreased FAS promoter activity. The −979/−997 STAT3 binding site of CPT Iα1b and the −794/−812 STAT3 binding site of FAS were functional binding loci responsible for leptin-induced transcriptional activation. The study provided direct evidence that STAT3 regulated the expression of CPT Iα1b and FAS at the transcription level, and determined the STAT3 response element on promoters of CPT Iα1b and FAS under leptin signal.

Multi-tissue transcriptomic study reveals the main role of liver in the chicken adaptive response to a switch in dietary energy source through the transcriptional regulation of lipogenesis

Background

Because the cost of cereals is unstable and represents a large part of production charges for meat-type chicken, there is an urge to formulate alternative diets from more cost-effective feedstuff. We have recently shown that meat-type chicken source is prone to adapt to dietary starch substitution with fat and fiber. The aim of this study was to better understand the molecular mechanisms of this adaptation to changes in dietary energy sources through the fine characterization of transcriptomic changes occurring in three major metabolic tissues – liver, adipose tissue and muscle – as well as in circulating blood cells.

Results

We revealed the fine-tuned regulation of many hepatic genes encoding key enzymes driving glycogenesis and de novo fatty acid synthesis pathways and of some genes participating in oxidation. Among the genes expressed upon consumption of a high-fat, high-fiber diet, we highlighted CPT1A, which encodes a key enzyme in the regulation of fatty acid oxidation. Conversely, the repression of lipogenic genes by the high-fat diet was clearly associated with the down-regulation of SREBF1 transcripts but was not associated with the transcript regulation of MLXIPL and NR1H3, which are both transcription factors. This result suggests a pivotal role for SREBF1 in lipogenesis regulation in response to a decrease in dietary starch and an increase in dietary PUFA. Other prospective regulators of de novo hepatic lipogenesis were suggested, such as PPARD, JUN, TADA2A and KAT2B, the last two genes belonging to the lysine acetyl transferase (KAT) complex family regulating histone and non-histone protein acetylation. Hepatic glycogenic genes were also down-regulated in chickens fed a high-fat, high-fiber diet compared to those in chickens fed a starch-based diet. No significant dietary-associated variations in gene expression profiles was observed in the other studied tissues, suggesting that the liver mainly contributed to the adaptation of birds to changes in energy source and nutrients in their diets, at least at the transcriptional level. Moreover, we showed that PUFA deposition observed in the different tissues may not rely on transcriptional changes.

Conclusion

We showed the major role of the liver, at the gene expression level, in the adaptive response of chicken to dietary starch substitution with fat and fiber.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4520-5) contains supplementary material, which is available to authorized users.

Seasoning ingredients in a medium-fat diet regulate lipid metabolism in peripheral tissues via the hypothalamic-pituitary axis in growing rats

We fed rats noodle (N) -diet containing 30 wt.% instant noodle with a 26% fat-to-energy ratio for 30 days (N-group). Compared with rats that were fed the same amount of nutrients (C-group), the N-group showed lower liver triacylglycerol levels and higher fecal cholesterol levels. We then analyzed transcriptome of the hypothalamic-pituitary (HP), the liver and the white adipose tissue (WAT). Thyroid stimulating hormone (Tshb), and its partner, glycoprotein hormone genes were up-regulated in the HP of N-group. Sterol regulatory element binding transcription factors were activated in the liver of N-group, while an up-regulation of the angiogenic signal occurred in the WAT of N-group. N-group showed higher urine noradrenaline (NA) level suggesting that these tissue signals are regulated by NA and Tshb. The N-diet contains 0.326 wt.% glutamate, 0.00236 wt.% 6-shogaol and Maillard reaction products. Our results suggest that these ingredients may affect lipid homeostasis via the HP axis.

The effect of thyroid dysfunction on nesfatin-1 and adiponectin levels in rats

Background Changes in thyroid hormone concentrations may affect adiponectin concentrations through various mechanisms. A molecule released primarily from the fat cells adiposities; adiponectin has important effects on the regulation of body weight. Aim The present study aimed to explore the effects of experimental thyroid dysfunction and its treatment on nesfatin-1 and adiponectin levels in rats. Methods The study included 40 adult male Sprague-Dawley rats which were grouped as follows: (1) control; (2) hypothyroidism [hypothyroidism was induced by intraperitoneal injection of 10 mg/kg/day propylthiouracil (PTU) for 3 weeks]; (3) hypothyroidism + thyroxine group [after hypothyroidism was induced by 2-week PTU injection, they were treated with high-dose L-thyroxine (1.5 mg/kg/day) for 1 week]; (4) hyperthyroidism [hyperthyroidism was induced by 3-weeks' thyroxine injection (0.3 mg/kg/day)]; (5) hyperthyroidism + PTU (after hyperthyroidism was induced by 2-weeks' thyroxine injection, the animals were given 10 mg/kg/day PTU for 1 week). Blood samples taken at the end of the study were analyzed to measure nesfatin-1 and adiponectin levels. Results It was found that nesfatin-1 levels increased in hypothyroidism, while adiponectin levels decreased (p < 0.001). In experimental hyperthyroidism, on the other hand, both nesfatin-1 and adiponectin levels were found significantly elevated (p < 0.001). Conclusion The results of the study indicate that nesfatin-1 and adiponectin levels were modified considerably in hypo- and hyperthyroidism, whereas with the restoration of the thyroid function, modified hormone levels went back to normal.

Protein profiling and functional analysis of liver mitochondria from rats with nonalcoholic steatohepatitis

Mitochondrial dysfunction is closely associated with the pathogenesis of nonalcoholic steatohepatitis (NASH). The aim of the present study was to comprehensively determine mitochondrial abnormalities in NASH by detecting the proteomics in liver mitochondria in a NASH rat model, which was induced for 16 weeks by the provision of a high fat and high cholesterol diet (HFD). Serum parameters, including triglycerides, total cholesterol, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol were determined, and hematoxylin and eosin staining of liver tissues was examined to evaluate the NASH rat model. Various parameters associated with mitochondrial function were examined, including mitochondrial DNA (mtDNA) copy number, mitochondrial membrane potential (MMP) and mitochondrial respiratory chain complex (MRC) activity. The mitochondrial proteomics were analyzed and identified using isobaric tags for relative and absolute quantitation labeling coupled with two-dimensional liquid chromatography-tandem mass spectrometry. The identified proteins were classified and grouped using the Blast2GO program against the non-redundant protein database, the Kyoto Encyclopedia of Genes and Genomes database and the Cluster of Orthologous Groups of proteins database. Compared with the control, mtDNA copy number, MMP, and activities of MRC I and III were decreased markedly in the HFD group. A total of 18 upregulated and 13 downregulated proteins were identified, with a significant 1.2-fold difference between the control and NASH groups. The dysregulated proteins were closely involved in mitochondrial oxidative phosphorylation, the lipid metabolic process and fatty acid β-oxidation. The results of the present study provide important proteomic information regarding liver mitochondria in NASH and serve as a basis for further detailed investigations of the pathogenesis of NASH.

Dysregulation of hepatic cAMP levels via altered Pde4b expression plays a critical role in alcohol-induced steatosis

Alcohol-induced hepatic steatosis is a significant risk factor for progressive liver disease. Cyclic adenosine monophosphate (cAMP) signalling has been shown to significantly regulate lipid metabolism; however, the role of altered cAMP homeostasis in alcohol-mediated hepatic steatosis has never been studied. Our previous work demonstrated that increased expression of hepatic phosphodiesterase 4 (Pde4), which specifically hydrolyses and decreases cAMP levels, plays a pathogenic role in the development of liver inflammation/injury. The aim of this study was to examine the role of PDE4 in alcohol-induced hepatic steatosis. C57BL/6 wild-type and Pde4b knockout (Pde4b(-/-) ) mice were pair-fed control or ethanol liquid diets. One group of wild-type mice received rolipram, a PDE4-specific inhibitor, during alcohol feeding. We demonstrate for the first time that an early increase in PDE4 enzyme expression and a resultant decrease in hepatic cAMP levels are associated with the significant reduction in carnitine palmitoyltransferase 1A (Cpt1a) expression. Notably, alcohol-fed (AF) Pde4b(-/-) mice and AF wild-type mice treated with rolipram had significantly lower hepatic free fatty acid content compared with AF wild-type mice. Importantly, PDE4 inhibition in alcohol-fed mice prevented the decrease in hepatic Cpt1a expression via the Pparα/Sirt1/Pgc1α pathway. These results demonstrate that the alcohol- induced increase in hepatic Pde4, specifically Pde4b expression, and compromised cAMP signalling predispose the liver to impaired fatty acid oxidation and the development of steatosis. Moreover, these data also suggest that hepatic PDE4 may be a clinically relevant therapeutic target for the treatment of alcohol-induced hepatic steatosis. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Farnesoid X receptor activation increases reverse cholesterol transport by modulating bile acid composition and cholesterol absorption in mice

Activation of farnesoid X receptor (FXR) markedly attenuates development of atherosclerosis in animal models. However, the underlying mechanism is not well elucidated. Here, we show that the FXR agonist, obeticholic acid (OCA), increases fecal cholesterol excretion and macrophage reverse cholesterol transport (RCT) dependent on activation of hepatic FXR. OCA does not increase biliary cholesterol secretion, but inhibits intestinal cholesterol absorption. OCA markedly inhibits hepatic cholesterol 7α-hydroxylase (Cyp7a1) and sterol 12α-hydroxylase (Cyp8b1) partly through inducing small heterodimer partner, leading to reduced bile acid pool size and altered bile acid composition, with the α/β-muricholic acid proportion in bile increased by 2.6-fold and taurocholic acid (TCA) level reduced by 71%. Overexpression of Cyp8b1 or concurrent overexpression of Cyp7a1 and Cyp8b1 normalizes TCA level, bile acid composition, and intestinal cholesterol absorption.

CONCLUSION

Activation of FXR inhibits intestinal cholesterol absorption by modulation of bile acid pool size and composition, thus leading to increased RCT. Targeting hepatic FXR and/or bile acids may be useful for boosting RCT and preventing the development of atherosclerosis. (Hepatology 2016;64:1072-1085).

Effects of pterostilbene in brown adipose tissue from obese rats

In recent years, much attention has been paid by the scientific community to phenolic compounds as active biomolecules naturally present in foods. Pterostilbene is a resveratrol dimethylether derivative which shows higher bioavailability. The aim of the present study was to analyze the effect of pterostilbene on brown adipose tissue thermogenic markers in a model of genetic obesity, which shows reduced thermogenesis. The experiment was conducted with 30 Zucker (fa/fa) rats that were distributed in three experimental groups: control and two groups orally administered with pterostilbene at 15 and 30 mg/kg body weight/day for 6 weeks. Gene expression of uncoupling protein 1 (Ucp1), peroxisome proliferator-activated receptor γ co-activator 1 α (Pgc-1α), carnitine palmitoyl transferase 1b (Cpt1b), peroxisome proliferator-activated receptor α (Pparα), nuclear respiratory factor 1 (Nfr1), and cyclooxygenase-2 (Cox-2); protein expression of PPARα, PGC-1α, p38 mitogen-activated protein kinase (p38 MAPK), UCP1 and glucose transporter (GLUT4); and enzyme activity of CPT 1b and citrate synthase (CS) were assessed in interscapular brown adipose tissue. With the exception of Pgc-1α expression, all these parameters were significantly increased by pterostilbene administration. These results show for the first time that pterostilbene increases thermogenic and oxidative capacity of brown adipose tissue in obese rats. Whether these effects effectively contribute to the antiobesity properties of these compound needs further research.

Carboxylesterase 2 prevents liver steatosis by modulating lipolysis, endoplasmic reticulum stress, and lipogenesis and is regulated by hepatocyte nuclear factor 4 alpha in mice

Nonalcoholic fatty liver disease (NAFLD) is a common liver disease that ranges from simple steatosis to nonalcoholic steatohepatitis (NASH). So far, the underlying mechanism remains poorly understood. Here, we show that hepatic carboxylesterase 2 (CES2) is markedly reduced in NASH patients, diabetic db/db mice, and high-fat diet (HFD)-fed mice. Restoration of hepatic CES2 expression in db/db or HFD-fed mice markedly ameliorates liver steatosis and insulin resistance. In contrast, knockdown of hepatic CES2 causes liver steatosis and damage in chow- or Western diet-fed C57BL/6 mice. Mechanistically, we demonstrate that CES2 has triglyceride hydrolase activity. As a result, gain of hepatic CES2 function increases fatty acid oxidation and inhibits lipogenesis, whereas loss of hepatic CES2 stimulates lipogenesis by inducing endoplasmic reticulum stress. We further show that loss of hepatic CES2 stimulates lipogenesis in a sterol regulatory element-binding protein 1 (SREBP-1)-dependent manner. Finally, we show that hepatocyte nuclear factor 4 alpha (HNF-4α) plays a key role in controlling hepatic CES2 expression in diabetes, obesity, or NASH.

CONCLUSION

CES2 plays a protective role in development of NAFLD. Targeting the HNF-4α/CES2 pathway may be useful for treatment of NAFLD. (Hepatology 2016;63:1860-1874).

(-)-Hydroxycitric acid reduced fat deposition via regulating lipid metabolism-related gene expression in broiler chickens

BACKGROUND

Chicken as a delicious food for a long history, and it is well known that excess fat deposition in broiler chickens will not only induced metabolic diseases, but also lead to adverse effect in the consumer's health. (-)-Hydroxycitric acid (HCA), a major active ingredient of Garcinia Cambogia extracts, had shown to suppress fat accumulation in animals and humans. While, the precise physiological mechanism of HCA has not yet been full clarified, especially its action in broiler chickens. Thus, this study aimed to assess the effect of (-)-HCA on lipid metabolism in broiler chickens.

METHODS

A total of 120 1-day-old broiler chickens were randomly allocated to four groups, with each group was repeated three times with 10 birds. Birds received a commercial diet supplemented with (-)-HCA at 0, 1000, 2000 or 3000 mg/kg, respectively, for a period of 4 weeks ad libitum.

RESULTS

Body weight (BW) in the 2000 and 3000 mg/kg (-)-HCA groups was significantly decreased (P < 0.05) than that in control group. A significantly decreased of serum triglyceride (TG) and density lipoprotein-cholesterol (LDL-C) content were observed in 3000 mg/kg (-)-HCA group (P < 0.05). Broiler chickens supplmented with 2000 and 3000 mg/kg (-)-HCA had pronouncedly higher hepatic lipase (HL) activity, hepatic glycogen and non-esterified fatty acid (NEFA) contents in liver (P < 0.05). Serum free triiodothyronine (FT3) and thyroxin (T4) contents were significantly higher in 3000 mg/kg (-)-HCA group (P < 0.05) compared with the control group. Supplemental (-)-HCA markedly decreased fatty acid synthase (FAS) and sterol regulatory element binding protein-1c (SREBP-1c) (P < 0.05) mRNA levels, while the mRNA abundance of adenosine 5'-monophosphate-activated protein kinaseβ2 (AMPKβ2) (P < 0.05) was significantly increased. In addition, ATP-citrate lyase (ACLY) mRNA level (P < 0.05) was significantly decreased in broiler chickens supplemented with 3000 mg/kg (-)-HCA. No differences was observed on carnitine palmitoyl transferase-I(CPT-I), while peroxisome proliferators-activated receptor α (PPARα) mRNA level (P < 0.05) was significantly increased in broiler chickens supplemented with 2000 and 3000 mg/kg (-)-HCA.

CONCLUSIONS

Supplemental (-)-HCA inhibited lipogenesis by inhibiting ACLY, SREBP-1c and FAS expression, and accelerated lipolysis through enhancing HL activity and PPARα expression, which eventually led to the reduced abdominal fat deposition in broiler chickens. Graphical abstract Mechanism of (-)-HCA effect on hepatic lipids metabolism.

The n-3 long-chain PUFAs modulate the impact of the GCKR Pro446Leu polymorphism on triglycerides in adolescents

Dietary n-3 long-chain PUFAs (LC-PUFAs) are associated with improvement in the parameters of the metabolic syndrome (MetS). Glucokinase regulatory protein (GCKR) is a key protein regulating intracellular glucose disposal. Our aim was to investigate: i) the relationship between the GCKR rs1260326 (Pro446Leu) polymorphism and parameters of the MetS; and ii) a potential influence of n-3 and n-6 LC-PUFA levels on this relationship in the HELENA study (1,155 European adolescents). Linear regression analyses were performed to study the association between rs1260326 and the outcomes of interest. Interactions between rs1260326 and LC-PUFA levels on outcomes were explored. The T allele of rs1260326 was associated with higher serum TG concentrations compared with the C allele. In contrast to n-6 LC-PUFA levels, a significant interaction (P = 0.01) between rs1260326 and total n-3 LC-PUFA levels on serum TG concentrations was observed. After stratification on the n-3 LC-PUFA median values, the association between rs1260326 and TG concentration was significant only in the group with high n-3 LC-PUFA levels. In conclusion, this is the first evidence that n-3 LC-PUFAs may modulate the impact of the GCKR rs1260326 polymorphism on TG concentrations in adolescents. Several molecular mechanisms, in link with glucose uptake, could explain these findings.

A metabolic stress-inducible miR-34a-HNF4α pathway regulates lipid and lipoprotein metabolism

Non-alcoholic fatty liver disease (NAFLD) is one of the most common liver diseases, but its underlying mechanism is poorly understood. Here we show that hepatocyte nuclear factor 4α (HNF4α), a liver-enriched nuclear hormone receptor, is markedly inhibited whereas miR-34a is highly induced in patients with non-alcoholic steatohepatitis, diabetic mice and mice fed a high fat diet. miR-34a is essential for HNF4α expression and regulates triglyceride accumulation in human and murine hepatocytes. miR-34a inhibits very low-density lipoprotein secretion and promotes liver steatosis and hypolipidemia in an HNF4α-dependent manner. As a result, increased miR-34a or reduced HNF4α expression in the liver attenuates the development of atherosclerosis in Apoe^−/− or Ldlr^−/− mice. These data indicate that the miR-34a-HNF4α pathway is activated under common conditions of metabolic stress and may have a role in the pathogenesis of NAFLD and in regulating plasma lipoprotein metabolism. Targeting this pathway may represent a novel approach for the treatment of NAFLD.

Ring finger protein20 regulates hepatic lipid metabolism through protein kinase A-dependent sterol regulatory element binding protein1c degradation

Sterol regulatory element binding protein1c (SREBP1c) is a key transcription factor for de novo lipogenesis during the postprandial state. During nutritional deprivation, hepatic SREBP1c is rapidly suppressed by fasting signals to prevent lipogenic pathways. However, the molecular mechanisms that control SREBP1c turnover in response to fasting status are not thoroughly understood. To elucidate which factors are involved in the inactivation of SREBP1c, we attempted to identify SREBP1c-interacting proteins by mass spectrometry analysis. Since we observed that ring finger protein20 (RNF20) ubiquitin ligase was identified as one of SREBP1c-interacting proteins, we hypothesized that fasting signaling would promote SREBP1c degradation in an RNF20-dependent manner. In this work, we demonstrate that RNF20 physically interacts with SREBP1c, leading to degradation of SREBP1c via ubiquitination. In accordance with these findings, RNF20 represses the transcriptional activity of SREBP1c and turns off the expression of lipogenic genes that are targets of SREBP1c. In contrast, knockdown of RNF20 stimulates the expression of SREBP1c and lipogenic genes and induces lipogenic activity in primary hepatocytes. Furthermore, activation of protein kinase A (PKA) with glucagon or forskolin enhances the expression of RNF20 and potentiates the ubiquitination of SREBP1c via RNF20. In wild-type and db/db mice, adenoviral overexpression of RNF20 markedly suppresses FASN promoter activity and reduces the level of hepatic triglycerides, accompanied by a decrease in the hepatic lipogenic program. Here, we reveal that RNF20-induced SREBP1c ubiquitination down-regulates hepatic lipogenic activity upon PKA activation.

CONCLUSION

RNF20 acts as a negative regulator of hepatic fatty acid metabolism through degradation of SREBP1c upon PKA activation. Knowledge regarding this process enhances our understanding of how SREBP1c is able to turn off hepatic lipid metabolism during nutritional deprivation.

Molecular pathways in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a clinicopathological change characterized by the accumulation of triglycerides in hepatocytes and has frequently been associated with obesity, type 2 diabetes mellitus, hyperlipidemia, and insulin resistance. It is an increasingly recognized condition that has become the most common liver disorder in developed countries, affecting over one-third of the population and is associated with increased cardiovascular- and liver-related mortality. NAFLD is a spectrum of disorders, beginning as simple steatosis. In about 15% of all NAFLD cases, simple steatosis can evolve into non-alcoholic steatohepatitis, a medley of inflammation, hepatocellular injury, and fibrosis, often resulting in cirrhosis and even hepatocellular cancer. However, the molecular mechanism underlying NAFLD progression is not completely understood. Its pathogenesis has often been interpreted by the “double-hit” hypothesis. The primary insult or the “first hit” includes lipid accumulation in the liver, followed by a “second hit” in which proinflammatory mediators induce inflammation, hepatocellular injury, and fibrosis. Nowadays, a more complex model suggests that fatty acids (FAs) and their metabolites may be the true lipotoxic agents that contribute to NAFLD progression; a multiple parallel hits hypothesis has also been suggested. In NAFLD patients, insulin resistance leads to hepatic steatosis via multiple mechanisms. Despite the excess hepatic accumulation of FAs in NAFLD, it has been described that not only de novo FA synthesis is increased, but FAs are also taken up from the serum. Furthermore, a decrease in mitochondrial FA oxidation and secretion of very-low-density lipoproteins has been reported. This review discusses the molecular mechanisms that underlie the pathophysiological changes of hepatic lipid metabolism that contribute to NAFLD.

Steatosis and steatohepatitis: complex disorders

Non-alcoholic fatty liver disease (NAFLD) which includes steatosis and steatohepatitis, in particular non-alcoholic steatohepatitis (NASH), is a rising health problem world-wide and should be separated from alcoholic steatohepatitis (ASH). NAFLD is regarded as hepatic manifestation of the metabolic syndrome (MetSy), being tightly linked to obesity and type 2 diabetes mellitus (T2DM). Development of steatosis, liver fibrosis and cirrhosis often progresses towards hepatocellular carcinogenesis and frequently results in the indication for liver transplantation, underlining the clinical significance of this disease complex. Work on different murine models and several human patients studies led to the identification of different molecular key players as well as epigenetic factors like miRNAs and SNPs, which have a promoting or protecting function in AFLD/ASH or NAFLD/NASH. To which extent they might be translated into human biology and pathogenesis is still questionable and needs further investigation regarding diagnostic parameters, drug development and a better understanding of the genetic impact. In this review we give an overview about the currently available knowledge and recent findings regarding the development and progression of this disease.

Thyroid hormone regulation of metabolism

Thyroid hormone (TH) is required for normal development as well as regulating metabolism in the adult. The thyroid hormone receptor (TR) isoforms, α and β, are differentially expressed in tissues and have distinct roles in TH signaling. Local activation of thyroxine (T4), to the active form, triiodothyronine (T3), by 5'-deiodinase type 2 (D2) is a key mechanism of TH regulation of metabolism. D2 is expressed in the hypothalamus, white fat, brown adipose tissue (BAT), and skeletal muscle and is required for adaptive thermogenesis. The thyroid gland is regulated by thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH). In addition to TRH/TSH regulation by TH feedback, there is central modulation by nutritional signals, such as leptin, as well as peptides regulating appetite. The nutrient status of the cell provides feedback on TH signaling pathways through epigentic modification of histones. Integration of TH signaling with the adrenergic nervous system occurs peripherally, in liver, white fat, and BAT, but also centrally, in the hypothalamus. TR regulates cholesterol and carbohydrate metabolism through direct actions on gene expression as well as cross-talk with other nuclear receptors, including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR), and bile acid signaling pathways. TH modulates hepatic insulin sensitivity, especially important for the suppression of hepatic gluconeogenesis. The role of TH in regulating metabolic pathways has led to several new therapeutic targets for metabolic disorders. Understanding the mechanisms and interactions of the various TH signaling pathways in metabolism will improve our likelihood of identifying effective and selective targets.

Increased hepatic yolk precursor synthesis, secretion and facilitated uptake by follicles are involved in the rejuvenation of reproductive performance of molted hens (Gallus gallus domesticus)

Molt, a natural behavior that is initiated at the end of a lay cycle in birds, is implicated in the regression of the reproductive system in birds followed by a rejuvenation of egg-laying potential. The aim of the present study was to evaluate the physiological basis for the apparent rejuvenation of egg production that occurs following molting. Eighty-three-week-old Hy-line hens, were obtained and subjected to forced molting. Blood and tissue samples were obtained at the beginning of molt (at 83 weeks of age), during molt (at 85 weeks of age) and postmolt (at 89 weeks of age). The laying performance, egg quality, blood parameters and gene expression in the liver and the ovary were investigated before, during and after molt. There was an obvious increase in the postmolt laying rate from 70% premolt to 93% postmolt. Eggshell thickness, albumin height, Haugh unit and egg shape index were all significantly improved after molt. The circulating levels of estrogen and progesterone were lower in the postmolt hens, whereas the concentrations of luteinizing hormone and follicle stimulating hormone were not significantly affected by molt. These results indicate that enhanced hepatic yolk precursor synthesis and secretion contribute to increased postmolt laying performance. Molt enhanced the sensitivity of sex hormones in F1 follicles. Augmented gene expression in the ovary was involved in the rejuvenation of the reproductive performance of molted hens. These results suggest that facilitated yolk-precursor uptake by follicles is involved in the rejuvenation of the reproductive performance of molted hens.

Non-classic thyroid hormone signalling involved in hepatic lipid metabolism

Thyroid hormones are important modulators of lipid metabolism because the liver is a primary hormonal target. The hypolipidaemic effects of thyroid hormones result from the balance between direct and indirect actions resulting in stimulation of lipid synthesis and lipid oxidation, which favours degradation pathways. Originally, it was believed that thyroid hormone activity was only transduced by alteration of gene transcription mediated by the nuclear receptor thyroid hormone receptors, comprising the classic action of thyroid hormone. However, the discovery of other effects independent of this classic mechanism characterised a new model of thyroid hormone action, the non-classic mechanism that involves other signalling pathways. To date, this mechanism and its relevance have been intensively described. Considering the increasing evidence for non-classic signalling of thyroid hormones and the major influence of these hormones in the regulation of lipid metabolism, we reviewed the role of thyroid hormone in cytosolic signalling cascades, focusing on the regulation of second messengers, and the activity of effector proteins and the implication of these mechanisms on the control of hepatic lipid metabolism.

Expression levels of candidate genes for intramuscular fat deposition in two Banna mini-pig inbred lines divergently selected for fatness traits

Intramuscular fat (IMF) content plays an important role in meat quality. Many genes involved in lipid and energy metabolism were identified as candidate genes for IMF deposition, since genetic polymorphisms within these genes were associated with IMF content. However, there is less information on the expression levels of these genes in the muscle tissue. This study aimed at investigating the expression levels of sterol regulating element binding protein-1c (SREBP-1c), diacylglycerol acyltransferase (DGAT-1), heart-fatty acids binding protein (H-FABP), leptin receptor (LEPR) and melanocortin 4 receptor (MC4R) genes and proteins in two divergent Banna mini-pig inbred lines (BMIL). A similar growth performance was found in both the fat and the lean BMIL. The fat meat and IMF content in the fat BMIL were significantly higher than in the lean BMIL, but the lean meat content was lower. The serum triacylglycerol (TAG) and free fatty acid (FFA) contents were significantly higher in the fat than in the lean BMIL. The expression levels of SREBP-1c, DGAT-1 and H-FABP genes and proteins in fat BMIL were increased compared to the lean BMIL. However, the expression levels of LEPR and MC4R genes and proteins were lower.

Elongation and desaturation pathways in mammary gland epithelial cells are associated with modulation of fat and membrane composition

The aim was to determine the relative role of each of the lactogenic hormones (insulin, prolactin and hydrocortisol) and their combinations in regulating elongation and desaturation of polyunsaturated fatty acids and subsequently on composition of cellular lipid compartments in mammary epithelia. Cultured cells of the mammary gland epithelial cell line HC11 were subjected to 48 h of hormonal treatment with different combinations of insulin, hydrocortisone and prolactin. Only the combination of all three hormones induced differentiation according to the marker β-casein gene expression. Inclusion of insulin in the treatment medium increased total fatty acid amount by 50% and increased the concentration of monounsaturated fatty acids by 12% while decreasing that of saturated fatty acids by 35%. Changes in the levels of fatty acids by chain length and saturation paralleled mRNA expression of the desaturases and elongases, whose expression levels were regulated again by inclusion of all three hormones in the treatment medium. Gene expression levels of the Δ6 desaturase and elongase 5 genes (Elovl 5) increased by approximately 1.5-fold, whereas expression of Elovl 4 decreased in the presence of all three hormones. Insulin was the main hormone inducing compositional differences in membrane lipids, increasing phosphatidylethanolamine and phosphatidylinositol and decreasing sphingomyelin and cholesterol. The results indicate that mammary gland epithelial cells express five out of the seven known elongase subtypes which are regulated primarily by the processes of differentiation and produce major compositional changes in mammary gland epithelial cells.

Fat metabolism is regulated by altered gene expression of lipogenic enzymes and regulatory factors in liver and adipose tissue but not in semimembranosus muscle of pigs during the fattening period

It has been shown previously that lipid metabolism is regulated by fatty acids (FA) and that thyroid hormones are important regulators of energy metabolism. The effects of weight, dietary fat level and dietary FA profile on thyroid hormone levels and expression of lipogenic genes and tissue FA composition were studied. Sixty-one crossbred gilts weighing 62 ± 5.2 kg BW average were either slaughtered at the beginning of the trial (n = 5) or fed one of seven diets (n = 8 pigs per diet): a semi-synthetic diet formulated to contain a very low level of fat (NF) and six diets based on barley-soybean meal supplemented with approximately 10% fat of different origin and slaughtered at 100 kg BW. The supplemental fats were tallow, high-oleic sunflower oil, sunflower oil (SFO), linseed oil, fat blend (55% tallow, 35% sunflower oil, 10% linseed oil) and fish oil blend (40% fish oil, 60% linseed oil). In general, the dietary FA profiles altered the FA composition of liver, semimembranosus muscle and adipose tissues. Pigs fed the NF diet had the highest free and total triiodothyronine (T3) values followed by pigs fed SFO. Total T3 levels were higher in pigs at 60 kg than in pigs at 100 kg. Correlations between thyroid hormones and genes encoding enzymes of fat synthesis in adipose tissue (acetyl CoA carboxylase (ACACA), fatty acid synthase and stearoyl CoA desaturase (SCD)) and the large differences in expression of lipogenic genes at different weights (60 and 100 kg BW), suggest a role for thyroid hormones and for T3, in particular, in regulating whole animal fat metabolism, with effects brought about by altered expression of lipogenic genes. Liver sterol receptor element binding protein-1 (SREBP1) mRNA content was affected by dietary treatment (P < 0.001) and was correlated with ACACA and SCD, whereas adipose tissue SREBP1 was not correlated with the mRNA abundance of any lipogenic enzyme. Weight and tissue factors showed greater influence on mRNA abundance of genes related with lipid metabolism than diet and tissue FA composition. In the pig, FA synthesis appear to be of greater magnitude in adipose tissue than in the liver as suggested by the higher expression of lipogenic genes in adipose tissue.

Expression of lipogenic genes during porcine intramuscular preadipocyte differentiation

Intramuscular fat (IMF) content plays an important role in meat quality. Triglyceride (TG) metabolism in intramuscular adipocytes is strongly associated with the intramuscular fat deposition. To better understand the mechanisms leading to IMF deposition we compared the expression levels of genes related to preadipocyte differentiation and lipogenesis in the intramuscular preadipocytes isolated from the longissimus muscle of Wujin and Landrace pigs. The results showed that the intramuscular preadipocytes could differentiate into mature adipocytes in vitro. Triglyceride content in adipocytes isolated from Wujin pigs was higher than Landrace pigs during the middle and later phases of preadipocyte differentiation. The expression levels of genes related to preadipocyte differentiation such as PPARG and CEBPA showed differential expression between Wujin and Landrace porcine adipocytes during the early stage of differentiation. The expression levels of lipogenic genes such as FASN and SREBF1 were significantly higher in Wujin porcine intramuscular preadipocytes than in Landrace intramuscular preadipocytes at the middle and the later stages of differentiation. This suggests that preadipocyte differentiation and lipogenesis exhibited breed-related scheduling.

3,5,3'triiodo-L-thyronine induces SREBP-1 expression by non-genomic actions in human HEP G2 cells

Liver is an important target for thyroid hormone actions. T(3) exerts its effects by two mechanisms: (i) Genomic actions consisting of T(3) link to nuclear receptors that bind responsive elements in the promoter of target genes, (ii) non-genomic actions including integrin αvb3 receptor-mediated MAPK/ERK and PI3K/Akt/mTOR-C1 activation. SREBP-1a, SREBP-1c, and SREBP-2 are transcription factors involved in the regulation of lipogenic genes. We show in Hep G2 cells that T(3) determined a dose- and time-dependent increase in the level of the precursor form of SREBP-1 without affecting SREBP-1 mRNA abundance. T(3) also induced phosphorylation of ERK1/2, Akt and of mTOR-C1 target S6K-P70, and the cytosol-to-membrane translocation of PKC-α. Modulation of SREBP-1 protein level by T(3) was dependent on MAPK/ERK, PI3K/Akt/mTOR-C1 pathway activation since the MEK inhibitor PD98059 or the PI3K inhibitor LY294002 abolished the stimulatory effect of T(3) . Conversely, the effect of T(3) on SREBP-1 level was enhanced by using rapamycin, mTOR-C1 inhibitor. These data suggest a negative control of mTOR-C1 target S6K-P70 on PI3K/Akt pathway. The effect of T(3) on SREBP-1 content increased also by using PKC inhibitors. These inhibitors increased the action of T(3) on Akt phosphorylation suggesting that conventional PKCs may work as negative regulators of the T(3) -dependent SREBP-1 increase. T(3) effects were partially abrogated by tetrac, an inhibitor of the T(3) -αvβ3 receptor interaction and partially evoked by T(3) analog T(3) -agarose. These findings support a model in which T(3) activates intracellular signaling pathways which may be involved in the increment of SREBP-1 level through an IRES-mediated translation mechanism.

Thyroid hormone receptor-α gene knockout mice are protected from diet-induced hepatic insulin resistance

Nonalcoholic fatty liver disease (NAFLD) is the most frequent chronic liver disease in the United States and is strongly associated with hepatic insulin resistance. We examined whether the thyroid hormone receptor-α (Thra) would be a potential therapeutic target to prevent diet-induced NAFLD and insulin resistance. For that purpose, we assessed insulin action in high-fat diet-fed Thra gene knockout (Thra-0/0) and wild-type mice using hyperinsulinemic-euglycemic clamps combined with (3)H/(14)C-labeled glucose to assess basal and insulin-stimulated rates of glucose and fat metabolism. Body composition was assessed by (1)H magnetic resonance spectroscopy and energy expenditure by indirect calorimetry. Relative rates of hepatic glucose and fat oxidation were assessed in vivo using a novel proton-observed carbon-edited nuclear magnetic resonance technique. Thra-0/0 were lighter, leaner, and manifested greater whole-body insulin sensitivity than wild-type mice during the clamp, which could be attributed to increased insulin sensitivity both in liver and peripheral tissues. Increased hepatic insulin sensitivity could be attributed to decreased hepatic diacylglycerol content, resulting in decreased activation of protein kinase Cε and increased insulin signaling. In conclusion, loss of Thra protects mice from high-fat diet-induced hepatic steatosis and hepatic and peripheral insulin resistance. Therefore, thyroid receptor-α inhibition represents a novel pharmacologic target for the treatment of NAFLD, obesity, and type 2 diabetes.

Dexamethasone-induced hepatic lipogenesis is insulin dependent in chickens (Gallus gallus domesticus)

Hepatic lipogenesis-induced de novo by glucocorticoids (GCs) is associated with the development of obesity and diabetes mellitus. The interaction of GCs and insulin in the regulation of hepatic lipogenesis remains unclear. The effect of exogenous GC administration on hepatic lipogenesis and fat deposition was studied in broiler chickens (Gallus gallus domesticus), and the role of insulin in the effect of GCs on hepatic lipogenesis was evaluated. Dexamethasone (DEX, 2 mg/kg body mass (BM)) administration for 3-d resulted in BM loss and increased liver and cervical adipose tissue mass compared to control and pair-fed counterparts. DEX treatment significantly (P < 0.05) increased plasma level of insulin in either the fed or fasting state, whereas plasma glucose level was only increased in the fed state. In fasted chickens, DEX treatment significantly (P < 0.01) upregulated the hepatic mRNA levels of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). In the fed state, the mRNA levels of ACC and FAS were not significantly influenced by DEX treatment, nor was FAS activity. In cultured primary hepatocytes, combined DEX and insulin significantly upregulated the transcription of the genes for FAS (1.34-fold) and malic enzyme (1.72-fold). By contrast, the expression of sterol response element-binding protein-1 (SREBP-1) was significantly upregulated by insulin (1.67-fold) regardless of DEX. In abdominal adipose tissue, DEX treatment had no significant (P>0.05) effect on the activities and transcription of FAS. The expressions of lipoprotein lipase and peroxisome proliferator-activated receptor-γ were not significantly (P>0.05) affected by DEX treatment in either the fasting or fed state. The results indicate that DEX increased hepatic de novo lipogenesis via the increased activity and expression of lipogenic enzymes. Insulin-activated gene expression for SREBP-1 is suggested to be involved in stress-augmented hepatic lipogenesis.

Cross-regulation of hepatic glucose metabolism via ChREBP and nuclear receptors

There is a worldwide epidemic of obesity and type 2 diabetes, two major public health concerns associated with alterations in both insulin and glucose signaling pathways. Glucose is not only an energy source but also controls the expression of key genes involved in energetic metabolism, through the glucose-signaling transcription factor, Carbohydrate Responsive Element Binding Protein (ChREBP). ChREBP has emerged as a central regulator of de novo fatty acid synthesis (lipogenesis) in response to glucose under both physiological and physiopathological conditions. Glucose activates ChREBP by regulating its entry from the cytosol to the nucleus, thereby promoting its binding to carbohydrate responsive element (ChoRE) in the promoter regions of glycolytic (L-PK) and lipogenic genes (ACC and FAS). We have previously reported that the inhibition of ChREBP in liver of obese ob/ob mice improves the metabolic alterations linked to obesity, fatty liver and insulin-resistance. Therefore, regulating ChREBP activity could be an attractive target for lipid-lowering therapies in obesity and diabetes. However, before this is possible, a better understanding of the mechanism(s) regulating its activity is needed. In this review, we summarize recent findings on the role and regulation of ChREBP and particularly emphasize on the cross-regulations that may exist between key nuclear receptors (LXR, TR, HNF4α) and ChREBP for the control of hepatic glucose metabolism. These novel molecular cross-talks may open the way to new pharmacological opportunities. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Effect of early feed restriction on hepatic lipid metabolism and expression of lipogenic genes in broiler chickens

The study was conducted to investigate the effect of early feed restriction (ER) on lipid metabolism and mitochondrial function in the liver of broiler chickens. Newly hatched broiler chickens were randomly allocated into control and ER group which was subjected to feed restriction with feed provided on alternate days from hatch to 14 days of age (14 d), followed by ad libitum feeding until the end of the experiment on 63 d. ER group exhibited significantly lower body weight throughout the experiment. Serum concentrations of total cholesterol (TC) and high density lipoprotein cholesterol (HDLC) were significantly higher in ER group at 14 d (P<0.05), and the higher serum TC level in ER group was also observed at 63 d. In contrast, the contents of triglyceride (TG), TC and lipoprotein lipase (LPL) activity in liver were significantly lower in ER group at 14 d (P<0.05). At 14 d no significant difference was detected for the mRNA expression of the acetyl-CoA carboxylase-α (ACC-α), carnitine palmitoyltransferase I (CPT-I), sterol regulatory element binding protein-1c (SREBP-1c) or peroxisome proliferator-activated receptors α (PPAR-α) between control and ER group. At 63 d ACC-α mRNA expression was significantly down-regulated accompanied with a significantly up-regulated CPT-ImRNA and a decreased tendency of SREBP-1c mRNA expression in ER group (P=0.09). Swollen mitochondria with fragmented and reduced cristae were observed in liver of ER group at 14 d. Meanwhile the inner mitochondria membrane viscidity increased and hepatic mitochondrial superoxide dismutase (SOD) activity decreased at 14 d. The results suggest that feed restriction at early postnatal stage may produce long-term effect on lipid metabolism of broiler chicken, probably through, at least in part, alterations in mitochondria morphology and function.

Genetic epistasis analysis of 10 peroxisome proliferator-activated receptor γ-correlated genes in broiler lines divergently selected for abdominal fat content

Chicken peroxisome proliferator-activated receptor γ (PPARγ), which is highly expressed in adipose tissues, is a key factor in fat accumulation in the abdominal fat pad. In this study, association and pairwise epistasis analyses were performed for all the polymorphisms detected in PPARγ and for 9 genes from PPARγ-correlated lipid metabolic pathways for abdominal fat weight (AFW) in 10th-generation populations of Northeast Agricultural University broiler lines divergently selected for abdominal fat content. Epistatic networks were then reconstructed with the identified epistatic effects. Single-marker association analyses showed that 5 of the 20 screened polymorphisms were significantly associated with AFW (P < 0.05), and CCAAT/enhancer-binding protein α (C/EBPα) c.552G>A was 1 of the 5 significant loci. Pairwise interaction analyses showed that 15 pairs of polymorphisms reached a significance level of P < 2.64 × 10(-4) (adjusted by Bonferroni correction) in the lean line, 41 pairs reached significance in the fat line, and 7 pairs reached significance in both lines. Interestingly, many other loci interacted with C/EBPα c.552G>A in both lines. In epistatic network analyses, C/EBPα c.552G>A seemed to behave as a hub for the epistatic network in both lines. All these results revealed that the genetic architecture of C/EBPα c.552G>A for AFW seemed to be an apparent individual main-effect QTL but that it could be dissected into a genetic epistatic network. Our results suggest that C/EBPα c.552G>A might be the most important locus contributing to phenotypic variation in AFW among all the polymorphisms detected in this study.

Inhibition of insulin and T3-induced fatty acid synthase by hexanoate

Fatty acid synthase (FAS) is responsible for the de novo synthesis of palmitate and stearate. This enzyme is activated by insulin and T(3), and inhibited by fatty acids. In this study, we show that insulin and T(3) have an inducing effect on FAS enzymatic activity, which is synergetic when both hormones are present. Octanoate and hexanoate specifically inhibit this hormonal effect. A similar inhibitory effect is observed at the level of protein expression. Transient transfections in HepG2 cells revealed that hexanoate inhibits, at least in part, FAS at a transcriptional level targeting the T(3) response element (TRE) on the FAS promoter. The effect of C6 on FAS expression cannot be attributed to a modification of insulin receptor activation or to a decrease in T(3) entry in the cells. Using bromo-hexanoate, we determined that hexanoate needs to undergo a transformation in order to have an effect. When incubating cells with triglyceride-hexanoate or carnitine-hexanoate, no effect on the enzymatic activity induced by insulin and T(3) is observed. A similar result was obtained when cells were incubated with betulinic acid, an inhibitor of the diacylglycerol acyltransferase. However, the incubation of cells with Triacsin C, a general inhibitor of acyl-CoA synthetases, completely reversed the inhibitory effect of hexanoate. Our results suggest that in hepatic cells, hexanoate needs to be activated into a CoA derivative in order to inhibit the insulin and T(3)-induced FAS expression. This effect is partially transcriptional, targeting the TRE on the FAS promoter.

Epidermis-type lipoxygenase 3 regulates adipocyte differentiation and peroxisome proliferator-activated receptor gamma activity

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) is essential for adipogenesis. Although several fatty acids and their derivatives are known to bind and activate PPAR gamma, the nature of the endogenous ligand(s) promoting the early stages of adipocyte differentiation has remained enigmatic. Previously, we showed that lipoxygenase (LOX) activity is involved in activation of PPAR gamma during the early stages of adipocyte differentiation. Of the seven known murine LOXs, only the unconventional LOX epidermis-type lipoxygenase 3 (eLOX3) is expressed in 3T3-L1 preadipocytes. Here, we show that forced expression of eLOX3 or addition of eLOX3 products stimulated adipogenesis under conditions that normally require an exogenous PPAR gamma ligand for differentiation. Hepoxilins, a group of oxidized arachidonic acid derivatives produced by eLOX3, bound to and activated PPAR gamma. Production of hepoxilins was increased transiently during the initial stages of adipogenesis. Furthermore, small interfering RNA-mediated or retroviral short hairpin RNA-mediated knockdown of eLOX3 expression abolished differentiation of 3T3-L1 preadipocytes. Finally, we demonstrate that xanthine oxidoreductase (XOR) and eLOX3 synergistically enhanced PPAR gamma-mediated transactivation. Collectively, our results indicate that hepoxilins produced by the concerted action of XOR and eLOX3 may function as PPAR gamma activators capable of promoting the early PPAR gamma-dependent steps in the conversion of preadipocytes into adipocytes.

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.